Typically reverse-osmosis membrane apparatuses comprise a plurality of tubular membranes positioned within a tubular, outer, pressure vessel adapted to withstand the higher pressures associated with reverse-osmosis processes, for example, over 500 psi, and typically up to 800 to 1200 psi or higher. The tubular membranes, often precast, but which also may be cast or formed in situ, are placed on a support or backing material within the outer tubes. In one technique, a precast, cellulose-acetate-type tubular membrane is placed within a porous carrier sheet and is pulled through the outer tube to its position within the outer tube. The tubular membrane may be separated from the outer tube by a permeable liner. The membrane then may be inserted either in a porous tube or in a nonporous tube, with a plurality of generally uniformly spaced holes therein.
In practice, groups of such outer tubes with tubular membranes; for example, up to about 1 inch in diameter, are placed in membrane modules, wherein a series of generally parallel arranged tubes; for example, 8 to 30, are positioned in a module support, and the ends of the tubes are connected by U-bend tubes, to provide for a serial connection of the tubes in the module. Due to the high pressure involved, the outer tubes and U bends are of metal, and the U bends may be cast inside another end element, and each U-bend flange is then bolted securely to the respective end of the membrane outer tube with appropriate rubber fittings to ensure a leak-free operation. Such modules of membrane tubes also may be used with some modification in connection with lower-pressure membrane operations, such as ultrafiltration processes.
In operation, a feed stream, such as a saline or brackish-water stream, or an aqueous stream containing low-molecular-weight salts, such as from a whey process, is introduced into the feed inlet of the module and into the inside of the membrane tube. The concentrated feed stream from the reverse-osmosis process is removed from the feed outlet of the module and is recovered or sent to the feed inlet of the next module for further process or treatment. The permeate stream passes through the tube, which may be porous or nonporous with holes therein, and drops into a shroud or into a cabinet from which the permeate is collected. In certain cases, the membrane is coated on a porous rod, and the permeate stream passes into the porous rod and to a collection device at one end of the rod.
It is desirable to permit ready access to the individual membrane tubes at one or both ends for periodic maintenance or replacement of the membrane tubes or associated components. In addition, it is also important to withdraw permeate efficiently and rapidly from each tube and to provide means to flush, such as by through-flushing, the permeate channel in the tube for cleaning purposes and to introduce and withdraw detergent cleaning solutions and biocide solutions. This is particularly useful in the treatment of process streams containing or derived from biodegradable material, such as whey process streams. Thus, it is desirable to provide for an easy and simple method and apparatus which provide for effective and rapid cleaning of the permeate channel.